The invention relates to conically scanning radar tracking systems and particularly to countercountermeasures for such systems.
In conically scanning tracking radar systems the return from the target is received through an antenna lobe which makes a small angle with the antenna axis and is rotated at constant speed about the antenna axis. This produces an amplitude modulation of the received signal at the rotational or lobing frequency for any target that is displaced from the extended antenna axis. The amplitude of the modulation is proportional to the magnitude of the displacement and its phase gives the direction of the displacement. After detection, the modulation is applied as an error signal to antenna aiming circuits which act to reduce the error signal to zero by bringing the extended antenna axis into coincidence with the target, thus locking on the target.
Any extraneous amplitude modulation of the return signal that reaches the antenna aiming circuits contributes to the error signal and, if the resultant signal is sufficiently different in amplitude or phase, or both, from the true error signal, can cause the antenna to break the tracking lock on the true target and seek a false target. The aiming circuit is usually preceded by a bandpass filter centered on the lobing frequency to exclude modulations at other than the lobing frequency. Also, conically scanning systems are usually of the LORO (scan-on-receive-only) type in order not to reveal the lobing frequency. Nevertheless, any extraneous modulation of the return signal which has a component at the lobing frequency can modify the error signal and possibly break the tracking lock. The usual countermeasure against conically scanning tracking radars is the imposition of an amplitude modulation at the lobing frequency on the return signal from the target. Target scintillation is another cause of extraneous amplitude modulation of the return signal that may interfere with the tracking function.